U.S. patent number 5,396,349 [Application Number 07/919,060] was granted by the patent office on 1995-03-07 for lateral and longitudinal chromatic dispersion correction in display systems employing non-conformal reflection holograms.
This patent grant is currently assigned to Pilkington P.E. Limited. Invention is credited to Anthony J. Kirkham, David G. Norrie, Martin D. Roberts.
United States Patent |
5,396,349 |
Roberts , et al. |
March 7, 1995 |
Lateral and longitudinal chromatic dispersion correction in display
systems employing non-conformal reflection holograms
Abstract
An optical system for a display having a combiner (1) with a
non-conformal reflection hologram and in which an intermediate
image (II) of a display source is formed at a location in the light
path to and spaced from the combiner (1), the system comprising at
least one diffractive element (5,7) disposed in the light path
between the display source (2) and the intermediate image (II) and
arranged to counter, at least partially, the chromatic dispersion
of the non-conformal reflection hologram (1).
Inventors: |
Roberts; Martin D. (North
Wales, GB), Kirkham; Anthony J. (North Wales,
GB), Norrie; David G. (North Wales, GB) |
Assignee: |
Pilkington P.E. Limited
(GB)
|
Family
ID: |
10698990 |
Appl.
No.: |
07/919,060 |
Filed: |
July 23, 1992 |
Foreign Application Priority Data
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Jul 25, 1991 [GB] |
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9116108 |
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Current U.S.
Class: |
359/14;
359/15 |
Current CPC
Class: |
G02B
5/32 (20130101); G02B 27/0103 (20130101); G02B
27/0172 (20130101); G02B 27/017 (20130101); G02B
2027/0107 (20130101); G02B 2027/0116 (20130101); G02B
2027/0132 (20130101); G02B 2027/0145 (20130101) |
Current International
Class: |
G02B
5/32 (20060101); G02B 27/01 (20060101); G02B
27/00 (20060101); G03H 001/00 (); G02B
005/32 () |
Field of
Search: |
;359/13,14,15 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0151455 |
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Aug 1985 |
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EP |
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0278395 |
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Aug 1988 |
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EP |
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0344810 |
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Dec 1989 |
|
EP |
|
0405540 |
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Jan 1991 |
|
EP |
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2161615 |
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Jan 1986 |
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GB |
|
2197728 |
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May 1988 |
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GB |
|
2213951 |
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Aug 1989 |
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GB |
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2240853 |
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Aug 1991 |
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GB |
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WO8805553 |
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Jul 1988 |
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WO |
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WO8912840 |
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Dec 1989 |
|
WO |
|
Other References
R N. Winner & J. H. Brindle, "A Holographic Visor
Helment-Mounted Display System", (a conference paper), 1974
Conference held by the IEEE, pp. 43-53. .
R. J. Withrington, "Optical Design of a Holographic Visor
Helmet-Mounted Display", SPIE vol. 147, Computer-Aided Optical
Design (1978), pp. 161-170..
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Primary Examiner: Ben; Loha
Assistant Examiner: Collins; Darryl J.
Attorney, Agent or Firm: Davis, IV; F. Eugene
Claims
We claim:
1. A display system comprising:
A) an illuminated image source providing non-monochromatic.
light;
B) a transparent non-conformal reflective holographic combiner
element;
C) means for directing light from said illuminated image source to
form a virtual image of said illuminated light source, and to be
reflected and affected at said holographic combiner element such
that said image has both lateral and longitudinal chromatic
dispersion; and
D) holographic means between said image source and said combiner
element for correcting both said lateral and longitudinal chromatic
dispersions wherein said holographic means takes the form of both
linear and circular gratings.
2. A display system comprising a display source adapted to issue
light from a display image, a lens arrangement adapted to form an
intermediate image of the display light, and a non-conformal
reflection hologram arranged to receive display light from the
intermediate image and to direct that light as a substantially
collimated beam towards an operator's station, wherein the hologram
forms part of an image combiner so that an observer located at said
observer's station views the image of the display in
superimposition with his view through the combiner, the display
source having a bandwidth that is not strictly monochromatic, and
longitudinal and lateral chromatic dispersion of the display light
generated by the hologram is countered, at least partially, by the
provision of finest and second mutually spaced diffractive elements
located in the light path between the display source and the
intermediate image, the first diffractive element being arranged
largely to counter lateral colour and basically taking the form of
a lined grating located adjacent the conjugate position of the
hologram with respect to the intermediate image, and the second
diffractive element being arranged largely to counter longitudinal
colour and basically taking the form of a circular grating located
adjacent a pupil position of the system.
3. A display system as claimed in claim 2, wherein the second
diffractive element takes the form of a surface relief hologram
formed on a surface of the lens arrangement.
4. A display system as claimed in claim 2, wherein the lens
arrangement comprises a relay lens having distributed elements and
the first and second diffractive elements are both located between
distributed elements of the relay lens.
5. A display system as claimed in claim 2, wherein the first and
second diffractive elements are combined into a single device
having the functions of linear and circular gratings, the device
being located sufficiently near to the said conjugate position to
provide an acceptable display image.
Description
This invention concerns improvements in or relating to displays and
relates more particularly to optical systems for displays having
combiners with non-conformal reflection holograms and displays
having such optical systems.
With some displays, such as for example head-up displays, an
observer, for example the pilot of an aircraft, looks through a
combiner which reflects display light towards him so that he sees
an image of the display superimposed on his view through the
combiner, e.g. of the outside scene. In avionic uses the light
reflected from the combiner is usually collimated so that the
display image appears at infinity. Some such displays are mounted
on a helmet worn by the observer. This can give rise to geometrical
configuration and weight problems in that axial or quasi-axial
combiner optical systems can be of considerable weight with most of
it in front of the observer's eyes. This is generally undesirable
in terms of user comfort and with aircraft pilots can be extremely
dangerous in high `g` and ejection situations. Off-axis systems can
use a mechanically supported tilted combiner which tends to
introduce undesirable obscurations and discontinuities in the field
of view of the outside world. Alternatively the helmet visor may be
used as the combiner by depositing a suitable coating as a
reflective patch on its inner surface and this can have major
advantages. Notably the lack of extra support structure reduces
weight and improves weight distribution by keeping the centre of
gravity back towards the centre of the observer's head and also
there is a better through-world view without discontinuities and
with good peripheral vision. It further has advantages in terms of
appearance, windblast and ease of manufacture. However, there are
problems from the optics aspect in that it is difficult to direct
the light to the observer's eyes with simple visor profiles and
adopting unduly complex visor profiles is not generally an
acceptable option. The physical geometry is not such as to permit
the required light direction by simple reflection from the visor
and it has therefore been proposed to put on the visor a
non-conformal reflection hologram, i.e. a hologram whose reflective
properties do not conform with the conventional laws of reflection
(by which the angle of incidence equals the angle of reflection)
but which effectively reflect light, by diffraction, in a direction
which deviates from that of simple reflection (so the angle of
reflection does not equal the angle of incidence). British Patent
No. 1 489 323 discloses such a system. While the non-conformal
reflection hologram approach may be useful with strictly
monochromatic display sources it runs into colour difficulties with
any appreciable display source bandwidth because of the high
dispersion of the hologram. While these difficulties have become
apparent particularly in the context of helmet mounted displays,
they could appear in other arrangements employing a non-conformal
reflection hologram on the combiner.
According to the present invention there is provided an optical
system for a display having a combiner with a non-conformal
reflection hologram and in which an intermediate image of a display
source is formed at a location in the light path to and spaced from
the combiner, the system comprising at least one diffractive
element disposed in the light path between the display source and
the intermediate image and arranged to counter, at least partially,
the chromatic dispersion of the non-conformal reflection
hologram.
The system may comprise two diffractive elements disposed in the
light path between the display source and the intermediate image,
one of which may be arranged largely to counter lateral colour and
the other of which may be arranged largely to counter longitudinal
colour. As explained later, colour dispersion by a diffractive
element is an angular effect but can be viewed as longitudinal
colour resulting from a finite pupil size and lateral colour
resulting from a finite image size (or finite field of view) and
the terms `lateral colour` and `longitudinal colour` are used
herein in that sense. The diffractive element which largely
counters lateral colour may basically take the form of a lined
diffraction grating and be located towards the conjugate position
of the non-conformal reflection hologram with respect to the
intermediate image. The diffractive element which largely counters
longitudinal colour may basically take the form of a circular
diffraction grating, preferably takes the form of a surface relief
hologram in a hybrid doublet (i.e. a combined refractive and
diffractive element), and may be located at or near a pupil
position. The two diffractive elements may be incorporated in a
relay lens which may incorporate a prism.
Alternatively in suitable circumstances the system may have a
single diffractive element disposed in the light path between the
display source and the intermediate image, which single diffractive
element may be located at or near the conjugate position of the
non-conformal hologram with respect to the intermediate image.
The invention further provides a display having an optical system
as set forth above, and particularly, but not exclusively, a helmet
mounted display in which the helmet visor carries the non-conformal
reflection hologram and constitutes the combiner. In a binocular
display two such systems may be used.
Embodiments of systems in accordance with the invention will now be
described, by way of example, with reference to the accompanying
drawings, in which:
FIGS. 1 and 2 are schematic representations illustrating certain
principles, and
FIG. 3 is a diagrammatic representation of a binocular system
including a ray trace.
FIG. 1 indicates geometrical considerations of the system. It has a
combiner in the form of a helmet visor 1 of spherical profile which
carries a non-conformal reflection hologram. An incident ray RI is
therefore reflected from the hologram not along the path RR which
would be taken if it conformed with the normal laws of reflection
(in which the angle between the incident and reflected rays would
be twice the angle of incidence) but, because of the diffractive
effect of the hologram, along a path RD (with the angle between the
incident and reflected rays less than twice the angle of
incidence). With this `sharper` reflection the ray RD is directed
to an eye position E1 of an observer in a manner which accommodates
the physical geometry of the arrangement. In FIG. 1 the incident
light ray RI is shown coming from a display source 2, for example a
cathode ray tube or liquid crystal display, via a lens 3, a mirror
4 and a diffractive element 5. The reason for the mirror 4 is
simply to bend the light path so as to provide physical space to
accommodate the observer's head. The function of these components
is further indicated in FIG. 2.
Referring to FIG. 2, the display source 2 is angled with respect to
the optical axis of the system in known manner and light emitted by
it passes to the lens 3. This lens is a hybrid doublet having both
refractive power and a diffractive effect. For convenience it can
be considered as a refractive lens 6 in combination with a
diffractive element 7 but in practice the diffractive element is a
surface relief hologram formed actually on the surface of the
refractive lens. Light transmitted through the lens 6 and
diffractive element 7 is reflected from the plane mirror 4 to the
diffractive element 5 and transmitted through the latter to form an
intermediate image II of the display. This intermediate image is at
a position in the light path to and spaced from the combiner 1 and
light from the image is reflected by diffraction from the
non-conformal hologram to the eye position E1 as previously
described. The overall optical power of the system is such that the
light reaching the observer's eye position E1 is substantially
collimated so that the observer can see an image of the display at
infinity superimposed on his view of the outside scene through the
combiner.
A diffractive element inevitably has high dispersion and the
non-conformal reflection hologram on the visor 1 therefore tends to
introduce an undesirable colour spread. A volume holographic
diffraction grating will result in typically 0.06 milliradian of
dispersion per nanometer per degree of non-conformity. Thus, for
example, 18.degree. of non-conformal tilt will result in about 10
milliradians of dispersion for a 10 nanometer waveband display
source phosphor. This is generally unacceptable in the viewed
image.
Since the combiner is displaced a long way from the system exit
pupil (typically one to two times the focal length) the dispersion
is a combination of chromatic aberration of principal rays,
referred to herein as lateral colour, and chromatic aberration of
aperture rays, referred to herein as longitudinal colour. As will
be understood by those skilled in the art, the colour spread can be
considered as having these two components, namely longitudinal
colour resulting from a finite pupil size and lateral colour
resulting from finite field angle. Both of these types of chromatic
aberration are in fact an angular effect but it is convenient to
view one as longitudinal colour in that the angular effect with the
finite pupil size spreads the colour along the optical axis.
The purpose of the two diffractive elements 5 and 7 in the system
is to counter the colour spread of the non-conformal hologram. To
this end the diffractive element 5 is conveniently a lined
diffraction grating which may be a conventional type ruled grating
or may be of holographic form. It will be understood that the line
spacing need not necessarily be constant. The grating is located in
similar space to, i.e. towards (but because of physical geometry
constraints not actually at) the conjugate or mirror image position
of, the combiner 1 with respect to the intermediate image II and
largely corrects lateral colour introduced by the hologram on the
combiner. It also serves to bend the optical axis which assists
configuration around the helmet shell. The bend, i.e. the
diffracted angle, is made such that the angular separation between
the zero and +1 orders (which is the desired or used one) is
sufficiently large such that the zero order is outside the field of
view. This means that appreciable correction needs to be given by
the grating hologram and dictates a reasonably large holographic
wedge in the combiner. The diffractive element 7 is an essentially
circular grating analagous to a zone plate and may be a blazed
binary element or a transmission hologram. For high efficiency it
may take the form of a blazed surface relief hologram on the lens
of the hybrid doublet. This diffractive element 7, which has very
low power, is located at or near a pupil position and largely
counters longitudinal colour (in the sense explained above). It
will be understood that there may be some functional overlap
between the diffractive elements 5 and 7 and that in combination
they introduce opposite dispersion to that introduced by the
non-conformal reflection hologram on the visor 1. Ideally they
completely compensate for the visor dispersion but in practice the
correction may be less than complete but nevertheless a useful
improvement providing an adequately colour corrected display
image.
FIGS. 1 and 2 show a second eye position E2 which can be fed by a
second system, the same as that described for the first eye
position E1 but of opposite hand effect. The systems are mounted on
the helmet worn by the observer and it will be noted that the
system (shown) carried on the left-hand side of the helmet serves
the right eye while the system (not shown) carried on the
right-hand side would serve the left eye. The respective
non-conformal reflective holograms are in the form of circular
patches suitably located on the spherical profile visor 1.
The binocular arrangement illustrated in FIG. 3 provides a helmet
mounted display with two systems (one for each eye) operating on
basically the same principles as described above with reference to
FIGS. 1 and 2. The FIG. 3 arrangement, however, has a visor 8 with
a central indent 9 so as to provide respective spherical profile
parts for the two eyes, each such part carrying its own
non-conformal reflective hologram. It will be understood of course,
that in FIG. 3 and in FIGS. 1 and 2 only that part of the visor in
front of the observer's eyes is shown and that in practice the
visor extends also round the sides of the observer's face in known
manner. Further in the FIG. 3 arrangement the right-hand optical
system serves the right eye while the left-hand optical system
serves the left eye (conversely to FIGS. 1 and 2). For convenience
only one optical system is described but the left and right-hand
systems carried on the helmet are the same but of opposite hand
(i.e. mirror images of each other with respect to the central plane
containing the indent 9).
Light emitted by a tilted display source 10 is received by a relay
lens shown as comprising tilted refractive lens elements 11 and 12,
a hybrid doublet 13 effectively having a diffractive element and a
refractive element, a shaped prism 14, a diffractive element 15 and
a refractive lens elements 16 and 17. It will be understood that
the precise design of the relay lens is within the competence of
those skilled in the art and that any suitable design may be used
employing spherical, toric, cylindrical and/or other aspheric
surfaces as required. It will further be understood that the hydrid
doublet 13 and diffractive element 15 in FIG. 3 correspond to, and
may take basically the same form as, hydrid doublet 6,7 and
diffractive element 5 in FIGS. 1 and 2. The prism 14 in FIG. 3
serves a similar path-bending or folding function to the mirror 4
in FIGS. 1 and 2 but the prism 14, being of a material of higher
refractive index than air, permits a shorter physical path length.
The side of the prism 14 opposite its internally reflecting plane
face is indented where the light is not required to travel to
provide physical space.
The lined grating diffractive element 15 is located immediately
behind the front lens group 16,17 of the relay lens and deviates,
bends or folds the optical axis through 27.degree. in a direction
away from the observer's head with benefits as previously mentioned
in relation to the element 5 of FIGS. 1 and 2. The hydrid doublet
13 consists effectively of a positive low dispersion crown
refractive lens element in front of a very high dispersion weak
negative diffractive component.
The relay lens produces an intermediate image II of the display and
the light is then reflected by diffraction from the non-conformal
hologram on the visor 8 to the eye position E2. The non-conformal
hologram gives a tilt or wedge of 18.degree. of the effective
reflecting plane so that the axis beam undergoes a deviation of
36.degree. in one plane from the conventionally reflected ray
direction. If desired it may also have some power to give better
compensation or trimming. The diffractive elements incorporated in
the relay lens serve to counter, i.e. correct or compensate for,
the colour spread of the non-conformal hologram as previously
described with reference to FIGS. 1 and 2, the lined grating
element 15 being located towards the conjugate position of the
non-conformal hologram with respect to the intermediate image and
largely correcting lateral colour and the circular grating element
in the hybrid doublet 13 being positioned at or near the pupil
immediately behind the prism 14 and largely correcting longitudinal
colour.
It will be appreciated that the helmet mounted display embodiments
specifically described above are given by way of illustration and
example and the invention can be used in other arrangements. In the
described helmet mounted arrangements it is not permissible for the
actual conjugate or mirror image position of the non-conformal
hologram with respect to the intermediate image to be occupied by
an optical element because of physical geometry constraints and in
particular because it would obstruct or interfere with the
observer's peripheral vision. In other arrangements where there is
no such constraint a diffractive element may be located at that
conjugate position and in that case a single diffractive element
may achieve substantially the same effect as the two diffractive
elements arranged in series as described above. Such single
diffractive element would generally be of more complex form than
either of the single grating elements since it would counter both
lateral and longitudinal colour. It would preferably be holographic
and of essentially opposite form to the non-conformal hologram on
the combiner. In some circumstances such single diffractive element
may be located not actually at but sufficiently near to the
conjugate position to give adequate colour correction and/or it may
be of a form which does not totally counter but sufficiently
corrects colour to give an acceptable display image.
It will further be appreciated that the invention is particularly
useful in situations having physical geometry constraints and is
potentially generally applicable to various displays having a
combiner, such as head-up displays for example, including those in
aircraft (e.g. aeroplanes and helicoptors) and ground vehicles
(e.g. cars and motor cycles) and whether monocular, biocular or
binocular, for military or civil use, and head (e.g. helmet) or
otherwise mounted. Yet further it will be understood that known
techniques of optimising light utilisation or minimising adverse
effects of unwanted light may be employed as appropriate in any
particular system. For example, masking or baffles may be used to
avoid ghosting through unwanted light transmitted at zero or other
unused orders by the diffractive element or elements. Further,
while the diffractive element or elements which counter the adverse
effects of the combiner non-conformal reflection hologram may, as
described above, be transmissive in mode, the single one or one or
both of the two could alternatively be of reflective mode.
* * * * *